In recent years, a liquid crystal display device, an organic EL display device, and the like have been used for a display device for a portable mobile device, a notebook computer, a television, and the like. These display devices are provided with an active matrix substrate as a semiconductor device. On the active matrix substrate, TFTs are formed. These TFTs switch voltages to be applied to pixel electrodes in a plurality of respective pixels that are disposed in a matrix. Further, on the active matrix substrate, TFTs of driving circuits are disposed on a periphery of a display region. These TFTs control a display voltage, a gate voltage, and the like that are applied to the plurality of pixels. For such TFTs, amorphous silicon TFTs, microcrystalline silicon TFTs, polycrystalline silicon (polysilicon) TFTs, and the like are used.
Patent Document 1 discloses an example of a semiconductor device that is provided with a plurality of TFTs. The semiconductor device disclosed therein is provided with two types of n-channel type TFTs and two types of p-channel type TFTs. These n-channel type TFTs and p-channel type TFTs respectively have active layers made of polycrystalline silicon. The two types of the n-channel type TFTs have two types of TFTs that respectively have threshold voltages (Vt) different from each other. Also, the two types of the p-channel type TFTs have two types of TFTs that respectively have threshold voltages different from each other. The channel region out of the two types of the n-channel type TFTs and the channel region out of the two types of the p-channel type TFTs are respectively doped with a dopant having the same concentration. The other channel region out of the two types of the n-channel type TFTs and the other channel region out of the two types of the p-channel type TFTs are set to non-doped regions that are not doped with a dopant.